Many multifunctional composite structures incorporate porosity at various length scales to increase the available surface area of a functional component. One material system of particular interest is activated or porous carbon fibers and nanofibers that can serve as structural reinforcement as well as providing active surface for added functionality. A key question in the design and manufacture of these fibers is to what degree the induced pore affects the mechanical properties by inducing discontinuities in the material. To address this problem, mechanics of porous carbon nanofibers (CNFs) was studied for the first time as a function of their porous structure. Hollow CNF with porous shell was prepared by coaxial electrospinning a polyacrylonitrile/poly(methyl methacrylate) (PMMA) blend shell with a PMMA core. PMMA was removed by thermal decomposition during pyrolysis to form pores. Solid-shell CNF was prepared as a control with no PMMA in the shell. Results show that the modulus and strength of the porous-shell CNF with a porosity of 19.2 ± 1.3% were 65.0 ± 6.2 and 1.28 ± 0.14 GPa respectively, 13.9 ± 2.1% and 35.5 ± 4.9% lower than those of the solid-shell CNF. Finite-element analysis models were developed to decouple the effect of stress concentration and reduced load-bearing area in porous CNFs on their mechanical properties. The model predictions were in general agreement with the experimental results and were used to identify the most critical parameters that can affect load bearing in porous nanofibers. Considering the comparison of the experimental and modeling results, the intrinsic material strength (of the solid parts) does not seem to be affected by inducing pores; thus, fiber and pore geometries might be developed where the load paths are designed for even less of a strength loss.
Keywords: carbon nanofibers; emulsion electrospinning; multifunctional; porous; stress concentration; structural energy storage.